The growth and development of physical fitness and the impact of undernutrition are being studied by measuring maximum oxygen consumption (VO2 max) in boys 6 to 16 years of age. Upper and lower socioeconomic groups from the urban setting of Cali, Colombia and a rural group of lower socioeconomic subjects from the environs of Cali are participating in the study. To date 505 subjects, approximately one half of the proposed number, have been studied. Preliminary results show marked stunting of growth in lower socioeconomic groups with resulting reduction in total VO2 max (L/min). These studies are continuing.